An electric double layer capacitor includes a capacitor element which has a pair of polarizable electrodes and a separator provided between the polarizable electrodes. The capacitor element is accommodated together with an electrolyte solution and sealed in a case. Each of the polarizable electrodes includes a collector. The electric double layer capacitor has higher output density than secondary batteries and can output a large current instantly, hence being used as a power assisting unit in electric vehicles and uninterruptible power sources. Electric double layer capacitors are demanded to have a large output density and an energy density, have reliability for continuously applying a voltage, and have durability to repetitive discharging and recharging cycles. Electric double layer capacitors particularly used for vehicles are demanded to have large output density and a large energy density.
The electric double layer capacitors store and release electric charges by electrolytic ions attached to and removed from electrodes, and hence, have a larger output density and better low-temperature characteristics than batteries with chemical reaction. The output density depends largely on an internal resistance and a voltage in the electric double layer capacitor. In order to increase the output density, the internal resistance is reduced. The internal resistance can be reduced by increasing the contact area between the electrode and the collector. In order to reduce the decreasing of the internal resistance with time, the joining strength between the electrode and the collector is necessary. In order to producing the large energy density, the thickness of the collector is decreased to allow the electrode to have the occupying rate of the electrode in a cell. In order to have a small thickness, the collector has a physical strength.
Patent document 1 discloses a collector made of an aluminum mesh and a collector made of an aluminum foil having roughened surfaces. Patent document 2 discloses a collector made of an aluminum expanded metal. Even these collectors do not provide the large contact area, the joining strength, or the small thickness of the collector enough for a particular application, such as vehicles. The collector made of the expanded metal which merely has apertures therein may increase its joining strength by widening the apertures to allow the two electrodes located on respective surfaces of the collector to be jointed. However, the widening of the apertures reduces the physical strength of the collector, accordingly preventing the collector from having a small thickness. Further, the widening of the apertures reduces a contacting area between the collector and the electrodes, accordingly increasing the internal resistance of the electric double layer capacitor.
Patent Document 1: Japanese Patent Laid-Open Publication No. 4-162510.
Patent Document 2: U.S. Pat. No. 4,327,400.